1. Field of the Invention
The invention relates to a separator for a non-aqueous electrolyte secondary battery, and a manufacturing method thereof.
2. Description of Related Art
A non-aqueous electrolyte secondary battery such as a lithium-ion secondary battery (lithium secondary battery) is lightweight and has high energy density compared to existing batteries, and has been used as a so-called portable power source for a personal computer (PC), a portable terminal, or the like, or a power source for driving a vehicle in recent years. Particularly, a lithium-ion secondary battery which achieves high energy density with a low weight is preferably used as a high-output power source for driving a vehicle such as an electric vehicle (EV), a hybrid vehicle (HV), or a plug-in hybrid vehicle (PHV). This type of secondary battery is typically constructed by accommodating an electrode body formed by laminating a positive electrode and a negative electrode with a separator interposed therebetween, in a case along with an electrolyte.
As the separator, a porous film made of a resin is typically used. The separator has a function of electrically insulating the positive and negative electrodes from each other, a function of holding a non-aqueous electrolyte, and a shutdown function (that is, a function of blocking a conducting path of charge carriers by softening when the inside of the battery is overheated and reaches a predetermined temperature range (typically the softening point of the separator)). Moreover, in addition to the above-described functions, the separator is required to have a role of preventing short-circuits caused by contact between the positive and negative electrodes (short-circuit prevention function) for the purpose of ensuring the safety of the battery and the device in which the battery is mounted. For example, when the inside of the battery is overheated to the softening point of the resin forming the separator or higher and the separator shrinks due to the heat, there is concern that a short-circuit may be generated due to an insufficient coating area of the separator over the electrodes, the breakage of the separator, or the like. Therefore, the separator is required to have performance in which an internal short-circuit is prevented by suppressing the shrinkage of the separator even in an environment at a high temperature, that is, a predetermined level of heat resistance (durability). As a method to meet the requirements, a configuration in which a porous heat resistance layer (HRL) is provided on the surface of a separator made of a resin is proposed. For example, in International Patent Publication No. WO2010/104127, a separator, in which a heat resistance layer primarily containing inorganic filler is provided on a substrate layer made of a resin and the peel strength between the substrate layer and the heat resistance layer is set to a predetermined strength or higher, and a battery provided with the separator, are described.
However, for example, in a case where the battery is exposed to harsher conditions (for example, exposure to an environment at a higher temperature, or exposure to an environment at a high temperature for a long period of time), the energy that causes the substrate layer to shrink becomes excessive, and there may be cases where the heat resistance layer peels off from the substrate layer. That is, although the heat resistance of the separator can be enhanced by forming the heat resistance layer on the surface of the substrate layer, when the heat resistance layer peels off from the substrate layer, it becomes difficult to suppress the shrinkage (thermal shrinkage) of the separator at a position where the heat resistance layer peels off, and there may be cases where the function of preventing short-circuits is not sufficiently exhibited.